Because of the risks from flu, they should be among the first in line to get both the H1N1 and seasonal influenza shots, medical experts say. Yet many are averse to vaccinations or medication.

As the H1N1 influenza vaccine trickles into clinics and pharmacies over the next few weeks, public health officials and doctors desperately hope that pregnant women will be at the front of the line for the shot. Past influenza pandemics have proved that they're at increased risk for severe complications -- and they appear to be even more vulnerable to this new flu strain.

On Thursday, a consortium of major medical groups, including the American Medical Assn., the Centers for Disease Control and Prevention, and the American College of Obstetricians and Gynecologists, released a statement pleading with pregnant women to get both the H1N1 and seasonal flu vaccines.

But pregnant women have a well-established antipathy toward vaccinations, with only 15% getting the flu vaccine in any given year, compared with 30% of the general population, according to CDC reports. The current shortage of the H1N1 vaccine may further stymie efforts at widespread immunization. Moreover, pregnant women distrust medications in general -- even though flu vaccines could prevent or mitigate infection should they be exposed.

All of these factors indicate doctors' hopes will go unrealized even as rates of flu-related deaths, hospitalizations and preterm births rise.

"We may have to hospitalize more pregnant women than we ever have before," said Dr. Allison Bryant, an assistant professor of obstetrics, gynecology and reproductive health at UC San Francisco, who has studied the current pandemic's effects on pregnant patients.

By Aug. 21, the latest date for which numbers are available, pregnant women accounted for 6% of the 484 confirmed U.S. deaths linked to what's known as swine flu, although they make up only 1% of the population. Two H1N1-related deaths in pregnant women have been reported in Los Angeles County, according to the county Department of Public Health.

As for pregnant women diagnosed with flu, of the 700 cases reported to the CDC, 14% required admission to an intensive care unit -- four times the hospitalization rate of non-pregnant women of similar age. That number is probably an underestimate, CDC officials said, because many cases aren't confirmed by testing.

Further, many pregnant women are teenagers or young adults -- among the groups most likely to develop severe cases of influenza.

"These otherwise healthy pregnant women can deteriorate rapidly," said Dr. Denise Jamieson, a medical officer in the CDC's division of reproductive health. "We have seen some cases where pregnant women present in the emergency room, and they are already so sick they are intubated and delivered very quickly."

Flu has long been known as a disaster for pregnant women. Complications and deaths from flu were much higher among pregnant women and newborns in the pandemics of 1918, 1957 and 1968 than among the general population. In the 1918 outbreak, 27% of flu infections in pregnant women were fatal, compared with about 10% of overall flu cases worldwide.

Unlike in previous pandemics, however, pregnant women have two options for protection this year: the H1N1 vaccine -- for which they and young children have been given priority -- and the antiviral medication Tamiflu.

"We usually say pregnant women should only take medications for which the benefits outweigh the risks," Bryant said. "But we certainly believe that is the case with H1N1."

Flu, in general, causes more complications in pregnancy because a woman's immune system is naturally weakened to prevent her body from rejecting the fetus. That boosts the chances of secondary infections, such as pneumonia.

In past pandemics, pregnant women had a 50% higher chance of developing pneumonia compared with other infected people, and such women were 50% more likely to die. Pneumonia poses a particular threat to pregnant women because they have limited lung capacity, and the H1N1 strain appears to settle deeper in the lungs of young people.

The risk of flu complications is highest for women infected during the third trimester, when lung capacity is most reduced because of pressure from the fetus. Pregnant women with asthma or other chronic health conditions are at especially high risk for complications -- as are those who are obese or poor, who are current or former smokers, or who have underlying heart disease.

Flu is not thought to be transmitted to the fetus, but the infection's indirect effects, such as high fever, can increase the risk of neural tube defects.

In past pandemics, flu during pregnancy has increased the rate of preterm birth. It also may increase the occurrence of miscarriages.

In the unusually severe 1918 pandemic, the miscarriage rate was an estimated 26% in women with uncomplicated flu and 52% in those with influenza and pneumonia, according to historical records. The rate of preterm birth was also about 50% in infected pregnant women.